The next generation mobile wireless communication system, which is referred to as “5G,” will support a diverse set of use cases and a diverse set of deployment scenarios. 5G will encompass an evolution of today's 4G networks and the addition of a new, globally standardized radio-access technology known as “New Radio” (NR).
The diverse set of deployment scenarios includes deployment at both low frequencies (100 s of MHz), similar to LTE today, and very high frequencies (mm waves in the tens of GHz). At high frequencies, propagation characteristics make achieving good coverage challenging. One solution to the coverage issue is to employ beamforming (e.g., high-gain beamforming) to achieve satisfactory link budget.
Beamforming (a.k.a., precoding) is an important technology in future radio communication systems. It can improve performance both by increasing the received signal strength, thereby improving the coverage, and by reducing unwanted interference, thereby improving the capacity. Beamforming can be applied both in a transmitter and a receiver.
In a transmitter, beamforming involves configuring the transmitter to transmit the signal in a specific direction (or a few directions) and not in other directions. In a receiver, beamforming involves configuring the receiver to receive signals from a certain direction (or a few directions) and not from other directions. When beamforming is applied in both the transmitter and the receiver for a given communication link, the combination of the beam used by the transmitter to transmit a signal to the receiver and the beam used by the receiver to receive the signal is referred to as a beam-pair link (BPL). Generally, the beamforming gains are related to the widths of the used beams: a relatively narrow beam provides more gain than a wider beam. A BPL can be defined for downlink (DL) and uplink (UL) separately or jointly based on reciprocity assumptions.
For a more specific description of beamforming, one typically talks about beamforming weights rather than beams. On the transmission side, the signal to be transmitted is multiplied with beamforming weights (e.g., complex constants) before being distributed to the individual antenna elements. There is a separate beamforming weight for each antenna element, which allows maximum freedom in shaping the transmission beam given the fixed antenna array. Correspondingly, on the receiving side, the received signal from each antenna element is multiplied separately with the beamforming weights before the signals are combined. However, in the context of the present text, the description is easier to follow if the somewhat simplified notion of beams, pointing in certain physical directions, is adopted.
Beamforming generally requires some form of beam management, such as beam search, beam refinement, and/or beam tracking, to determine what UL and/or DL transmit (TX) and receive (RX) beams to use for communication between two units. Typically, the two units are a radio access network (RAN) transmission and reception point (TRP) (e.g., a base station) and a user equipment (UE) (i.e., a device, such as, for example, a smartphone, a tablet, a sensor, a smart appliance (or other Internet-of-Things (IoT) device), etc., that is capable of wireless communication with a TRP).
Generally, beam search is used to discover and maintain BPLs. A BPL is expected to be discovered and monitored by the network using measurements on downlink reference signals used for beam management, such as channel state information (CSI) reference signals (CSI-RS). The CSI-RS for beam management can be transmitted periodically, semi-persistently or aperiodic (such as being event triggered) and they can be either shared between multiple UEs or be UE-specific. In order to find a suitable TRP TX beam the TRP transmits CSI-RS in different TRP TX beams on which the UEs performs reference signal received power (RSRP) measurements to produce a CSI-RSRP value, as is well known in the art, and reports back the N best TRP TX beams (where the value of N can be configured by the network). Furthermore, the CSI-RS transmission on a given TRP TX beam can be repeated to allow the UE to evaluate suitable UE RX beams, thus enabling so-called UE RX beam training. The TRP can setup a beam training procedure with a UE by transmitting a beam training configuration.
Multi-user Multiple-Input Multiple-Output (MU-MIMO) is foreseen to be a key technical component in 5G. The purpose of MU-MIMO is to serve multiple UEs simultaneously in the same time, frequency, and code resources and in this way increase the capacity in the system. It is expected that proper beamforming settings at the UE largely can improve the performance for MU-MIMO.